This invention relates to organic conductors and semiconductors which fall into the group of polymeric conductors. As is well known, such conducting polymers appear in some respect like common synthetic resinous materials, but unlike such common materials (plastics), conducting polymers defy conventional melt-processing, cannot be compacted, whether molded or extruded, in the usual ways, nor deposited as a continuous film from solution, and are far from stable in air even at ambient temperature conditions. As long as a polymer is formed by electrodeposition on an electrode its conductivity may be said to be fair depending upon the particular application for which the polymer is sought. But a polymer which defies compaction into a shaped article, places severe limitations upon its use. Because a compactable conductor (the term "conductor" as used herein includes semiconductors) is far more versatile in its applications, the problem was to find a compactable polymer.
Tinkering with the structure of conducting polymers to improve their processability, for example by introducing substituents, generally results in degradation of their already low conductivity, consistent with the belief that conductivity is along the polymer chains. Low conductivity in the range from about 10.sup.-5 to about 10.sup.-2 ohm.sup.-1 cm.sup.-1 (reciprocal ohms/cm is hereafter designated "S/cm" for convenience) places a conductor in the category or semiconductors, while conductivity in the range from about 10.sup.-2 to about 10.sup.2 S/cm and above places it in the category of relatively good conductors. Of course, such "low conductivity" is referred to as such only in relation to the high conductivity of metals, but this low conductivity is sufficiently high for a variety of applications, for example, as polymer films on electrodes, as is described in articles titled "Polymer Films on Electrodes. 6. Bioconductive Polymers Produced by Incorporation of Tetrathiafulvalenium in a Polyelectrolyte (Nafion) Matrix" by Henning, T. P., White, H. S., and Bard, A. J., J. Am. Chem. Soc., 103 3937-38, (1981); and, "Polymer-Modified Electrodes: A New Class of Electrochemical Materials", by Kaufman, F. B., Schroeder, A. H., Engler, E. M., and Patel, V. V. Appl. Phys. Lett., 36(6), 422-5, (1980).
Poly(2,5-pyrrole), also referred to herein as "PP" for brevity, in which the --NH-- group links sequences of conjugated double bonds, is normally an insulator, that is, has a conductivity less than about 10.sup.-10 ohm.sup.-1 cm.sup.-1 and is totally insoluble in known solvents. It is known however, that electrochemically polymerized PP has good conductivity, but coupled with its melt-processing-resistance and the poor integrity of PP film so formed, the metamorphosis of PP into a practical organic polymer conductor poses a formidable problem. Moreover, it is generally known that providing substituents on the pyrrole monomer does not improve the conductivity of PP. This is not undesirable with respect to tailoring a semiconductor but contraindicates a logical course of action for tailoring a relatively good conductor.
The interest in modification of electrode surfaces by covalently attaching an organic monolayer or by depositing a polymer film spurred the electrochemical polymerization of pyrrole under controlled conditions as reported in "Electrochemical Polymerization of Pyrrole" by Diaz, A. F. et al in J. C. S. Chem. Comm. 1979, 14, 635. The films may be prepared in a variety of aprotic solvents but are totally insoluble in known solvents including acetonitrile (MeCN), methylene chloride and propylene carbonate. Subsequently, PP with p-type conductivity of 100 S/cm was prepared which were stable in air. These films were prepared from MeCN solution using a tetraethylammonium tetrafluoroborate electrolyte, as described in "Organic Metals: Polypyrrole, a Stable Synthetic `Metallic` Polymer" by Kanazawa, K. K. et al in J. C. S. Chem. 1979, 15, 854. Because the polymer film remains on the electrode surface as it is generated, the ability of the PP film to conduct is critical for the continuation of the reaction forming the film. Considerations related to the forming of the film, the electroactive behavior of thin films, and other details are discussed in "Electrochemical Preparation and Characterization of Conducting Polymers" in Chemica Scripta., 1981, 17 145-148.
Particularly noteworthy is that PP requires no dopant because it is naturally positively charged indicating it already has an electron removed during polymerization. Even more noteworthy is that PP films which were formed with various substituents on the N-atom were also totally insoluble. The magnitude of this limitation may only be gauged in terms of the limited application of any polymer which requires that it be electrodeposited on an electrode as a film, and which must be used in no other but the film form. At best, film of known PPs is difficultly powdered, and such powders as are formed cannot be pressed into a coherent shaped article even at 100,000 psi.
Despite knowing that substituents on the pyrrole ring would diminish conductivity, and recognizing that the 2- and 5-positions must necessarily remain open if the substituted pyrrole is to be polymerized, I surmised that the possibility of making compactable PP might hinge upon my finding the correct combination of substituents on the pyrrole nucleus. I further hoped that such a 3- and/or 4-substituted pyrrole would lend itself to electrochemical polymerization with an appropriate electrolyte which might favorably affect the solubility of the polymer formed.
Since only the polymer is electrically conductive, it is essential that the substituted pyrrole be electrochemically polymerizable. It is known however, that certain substituents negate such polymerization. Thus, until a soluble polymer was actually prepared, there was no way of inferring which substituents were more desirable than others. It was evident to me that such a search for soluble PP would require the synthesis of a large number of substituted pyrroles, and, following their polymerization to PPs, these PPs would need to be culled for desirable properties.
Where one of the substitutable positions (namely either the 3- or 4-positions, the others being necessarily kept open for polymerization), is substituted with phenyl, the other has been substituted with a variety of substituents forming (i) 3,4-diphenylpyrrole (see CA 87 (13):102110z); CA 80 (17):95696v; CA 62:16251/C; (ii) 1-(4-phenyl-1H-pyrrol-3-yl)-ethanone (see CA 78(17):111044x; (iii) phenyl(4-phenyl-1H-pyrrol-3-yl)-methanone (see CA 91(7):56973r; or, (iv) the methyl ester of 1H-pyrrole-3-carboxylic acid (see CA 95(21):187068e; CA 95(17):150318k; or, (v) 3-cyano-4-phenylpyrrole (see CA 93(11):114314f; or, (vi) 3-chloro-4-phenyl-pyrrole (see CA 74(21):108594p; CA 73(21)10661n; CA 67(19):906-66g; or, (vii) 3-nitro-4-phenyl-pyrrole; or, (viii) 1-methyl-3-phenyl-4-[(trifluoromethyl)sulfonyl-]H-pyrrole (see CA 95(7):6)902d; or, (ix) 1,3-diphenyl-4-[(trifluoromethyl)sulfonyl]-1H-pyrrole (see CA 95(7):6]902d. However, PPs derived from such phenyl substituted monomers, even if electropolymerizable, are not compactable.
The only compound made with an ether substituent in the 3-position is the methoxy-phenyl pyrrole derivatives disclosed in "Synthesis of 3-substituted Pyrrole Derivatives with Anti-inflammatory Activity" by Kazuo Sakai et al, Chem. Pharm. Bull., Vol 28, 8, pp 2384-2393 (1980). In this procedure, an isocyano group is used, in addition to an amide group, to counteract the electron-donating effect of the ether substituent. Even so, upon closing of the pyrrole ring, amide substituents are present at both the 2- and 4-positions. These substituents may be removed by hydrolysis to the acid with NaOH, then decarboxylating with ethanolamine or glycerine. However, if the methyl substituent is desired at the 4-position, reduction will produce methyl substituents at both the 2- and 4-positions, with no known means for removing the 2-methyl selectively. In other words, though the electron-withdrawing power of the isocyano group was bolstered with an amide group to make the reactant a Michael acceptor, a substituent at the 4-position may not be made with the Sakai procedure if the 2-position is to be kept open.
Since the product of interest is the PP polymer, it is necessary that the N-adjacent carbon atoms (that is, the C atoms in the 2- and 5-positions) be left open. Of the remaining 3-, 4- and N-positions, there was nothing to suggest that the substituent on one of them would be critical insofar as being effective to transform incompactable PP to compactable PP, if indeed the substituted PP proved to be electrically conductive. Nor was there anything to suggest that the critical ether substituent in the 3- or 4-position might, with the proper choice of electrolyte, lend the PP both solubility and enhanced conductivity.
I polymerized several compounds I made for the specific purpose of studying the electronic effects of substituents on conductivity, and it was only by chance that I discovered the criticality of the substituents at the substitutable positions of a pyrrole monomer. This led me to find an appropriate method of preparing particularly substituted monomers suitable for electropolymerization.
Accordingly, I prepared a host of substituted pyrroles using known techniques of synthesis, and particularly (i) the van Leusen procedure (see "A New and Simple Synthesis of the Pyrrole Ring System from Michael Acceptors and Tosylmethylisocyanides" by van Leusen, A. M. et al, Tetrahedron Letters, No. 52, pp 5337-5340, 1972); (ii) the Anderson/Loader synthesis Tetrahedron Letters, Vol 22, No. 49 pp 4899-4900 (1981); (iii) the Rokach synthesis, Tetrahedron Letters, Vol 22, No. 49 pp 4901-4904 (1981); and (iv) the Baldwin synthesis J. Chem. Soc., Chem. Comm., pg 624 (1982).
I was unable to make substituted pyrroles with either a hydroxyphenyl substituent, or an ether substituent at either the 3- or 4-positions, by using either the Anderson/Loader, Rokach or Baldwin syntheses. And I did not expect to be able to make such a substituted pyrrole with the van Leusen procedure because it is well known that an ether and hydroxyl substituent are each electron-donating groups (see Physical Organic Chemistry, by Hammett, L. P., 2d ed. McGraw Hill & Co., New York 1970; and, "An Extended Table of Hammett Substituent Constants Based on the Ionization of Substituted Benzoic Acids", by McDaniel and Brown, J. Org. Chem. 23 420, 1958). The presence of the ether group, or the hydroxyphenyl group, would be expected to negate the necessary electron-withdrawal from between the vinylene C atoms of an alpha,beta-unsaturated ketone, ester or nitrile, without which closing of the pyrrole ring would not occur.
Further, I was aware that a "Michael condensation" between isopropyl p-methoxybenzylidenemethyl ketone with diethylmalonate, when carried out in ethanol as a solvent, gives p-methoxycinnamic acid, indicating a "reverse" or "retrograde reaction". See "The Michael Reaction", Organic Reactions Vol. 10, 187 et seq., John Wiley & Sons (1959).
Having succeeded in making the desired substituted pyrroles either with a ketone, or a nitrile, or an ester reactant, each of which was not a typical Michael acceptor, I electrochemically polymerized the hydroxyphenyl- and ether-substituted pyrroles so formed. Finally, I tested the PPs for electrical properties, and most of all, for compactability, and/or solubility in available solvents.
This invention is the culmination of that search.